Diabetes refers to a disease derived from multiple causative factors and characterized by elevated levels of plasma glucose or hyperglycemia, along with sugar, fat and protein metabolism disorders caused by insulin secretion and/or the action defects. Diabetes is an ancient disease due to the human body absolute or relative lack of insulin, which results in increased concentrations of glucose in the blood. The glucose is largely discharged in urine. High blood levels of glucose can cause several problems, such as excessive thirst, frequent urination, increased appetite, weight loss, dizziness, fatigue, and other symptoms.
Dipeptidyl peptidase-IV (DPP-IV) is a serine protease that cleaves N-terminal dipeptides from a peptide chain containing, preferably, a proline residue in the penultimate position. Although its biological role in mammalian systems has not been completely established, DPP-IV is believed to play an important role in neuropeptide metabolism, T-cell activation, attachment of cancer cells to the endothelium, and the entry of HIV into lymphoid cells (WO98/19998).
More recently, it was discovered that DPP-IV is responsible for inhibiting the secretion of glucagon-like peptide (GLP)-1. More particularly, DPP-IV cleaves the amino-terminal His-Ala dipeptide of GLP-1, degrading active GLP-1(7-36)NH2 into inactive GLP-1(9-36)NH2 (Endocrinology, 1999, 140: 5356-5363). Under the physiological condition, the half-life of the whole GLP-1 in blood circulation is short, the inactive metabolite from GLP-1 degraded by DPP-IV can combine with GLP-1 receptor to antagonize the active GLP-1, so the physiological response to GLP-1 is shortened. The endogenous even exogenous GLP-1 can be entirely protected by the DPP-IV inhibitor from being deactivated by DPP-IV, and the GLP-1 bioactivity can be significantly increased (5- to 10-fold). Since GLP-1 is a major stimulator of pancreatic insulin secretion and can directly effect on glucose disposal, the DPP-IV inhibitor is well useful for treating non-insulin-dependent diabetes mellitus (NIDDM) (U.S. Pat. No. 6,110,949).